To me the most important objection is that an email is a record of something, and needs to be self-contained and immutable for that reason.
When I get an email, I want to know that I can always come back to that exact email for reference, and that there's no way that it can have changed, or that the important information is externally referenced (and therefore also subject to change).
I think this is one important reason that more and more emails are just links to some website with the information on it (often with a login required as well). It allows the company sending you the email to retain control of that information. If you email me a text or PDF invoice, I can always come back to it for my own reference. If you send me a link to one, there's no guarantee I can still access it later.
When I get an email, I want to know that I can always come back to that exact email for reference, and that there's no way that it can have changed, or that the important information is externally referenced (and therefore also subject to change).
I think this is one important reason that more and more emails are just links to some website with the information on it (often with a login required as well). It allows the company sending you the email to retain control of that information. If you email me a text or PDF invoice, I can always come back to it for my own reference. If you send me a link to one, there's no guarantee I can still access it later.